The present disclosure relates to an image forming apparatus provided with a plurality of developing devices.
An image forming apparatus adopting an electrophotographic method such as a copier, a printer or a facsimile machine forms a toner image on an image carrier (e.g. photoconductive drum or transfer belt) by supplying toner to an electrostatic latent image formed on the image carrier to develop the electrostatic latent image. A touch-down development method using a two-component developer containing nonmagnetic toner and magnetic carrier is known as one of methods for performing the above development. In this case, a two-component developer layer (so-called magnetic brush layer) is carried on a magnetic roller, the toner is transferred from the two-component developer layer onto a developing roller and a toner layer is carried on the developing roller. Further, the electrostatic latent image is visualized by the supply of the toner from the toner layer to the image carrier. Conventionally, there has been known a technology on a leakage detecting operation for detecting a leakage voltage, at which leakage occurs, by changing inter-peak voltages of alternating-current voltages in a developing device adopting the touch-down development method.
The technology of such a leakage detecting operation is applicable to a two-component development method. In the two-component development method, if inter-peak voltages of alternating-current voltages are set to be larger, toner becomes cloudy in a development area as in a nonmagnetic one-component development method and the touch-down development method. Thus, development performance is enhanced. Particularly, the two-component development method in which a magnetic brush made of a thin layer slightly comes into contact with an image carrier is useful since toner can be in a powder cloud state.